Saros cycle
An eclipse cycle of about 18 years and 11 days, after which the Sun, Earth and Moon return to nearly the same arrangement.
Eclipses happen only when a new or full moon falls close to one of the two points where the Moon's tilted orbit crosses the plane of Earth's orbit. Three different lunar rhythms are involved: the month of phases, the month of crossing those points, and the month of the Moon's changing distance. The Saros cycle is a period after which all three line up again almost exactly.
It lasts about 6,585.3 days, or 18 years, 11 days and 8 hours. In that time there are 223 months of phases, 242 of crossings and 239 of changing distance, all matching to within a few hours. So an eclipse is followed one Saros later by another of very similar type and length.
The extra 8 hours matter. Earth turns an extra third of a rotation, so each eclipse in a series happens about 120 degrees of longitude farther west. After three cycles, 54 years and 34 days, an eclipse returns to roughly the same region. A complete series lasts 1,226 to 1,550 years and contains 69 to 87 eclipses.
Babylonian astronomers noticed that lunar eclipses repeat on this cycle, and the ancient Greek Antikythera mechanism tracked it with a dial driven by gears. The name comes from a mix-up: Edmond Halley adopted it in 1691 from an old source that had misapplied a Babylonian term.
The Saros tells you when a similar eclipse will recur; exactly where totality falls needs detailed calculation.
Coming up in future stories
- The 2,000-Year-Old Eclipse Computer c. 150–100 BC · Antikythera, Greece